Various embodiments of hydraulic device are described herein. IN particular, the embodiments described herein relate to an improved hydraulic device having an inlet side and an outlet side, the hydraulic device including two meshing cogwheels.
Generally, cogwheels in a gear run distinctly more quietly if, apart from the quality of the cogwheel and a good mounting (distance between axles, bearing play, etc.), as great an overlap ratio as possible is achieved. Therefore, attempts are made to use devices in which at least two teeth of one cogwheel are always in engagement simultaneously with two teeth of the other cogwheel during the rotation of the meshing cogwheels.
In addition to optimizing the noise, the efficiency is of crucial importance in hydraulic external cogwheel pumps. In order to achieve a good mechanical and volumetric efficiency, the external diameter of the cogwheels and the distance between their axles are to be selected so that an optimum ratio of cogwheel diameter to (radial) tooth length is guaranteed. This leads to designing the external diameter of the cogwheels so as to be small. However, a small external diameter of the cogwheels limits the maximum number of teeth. In cogwheels with straight teeth, the small number of teeth does not allow a permanent contact in many cases between two pairs of teeth. In order to nevertheless make a double contact possible, it is therefore necessary to provide oblique teeth having a sufficient inclination of the teeth. The advantages of oblique teeth compared with straight teeth additionally include smoother running and a smaller noise development, because each pair of teeth runs with a continuous transition in and out of engagement and therefore the transmission of the torque runs more smoothly. In addition, a greater force can be transmitted compared with a straight toothed wheel of the same size, because the working surfaces of the teeth are larger. However, it is to be noted that with greater angles of inclination, the axial forces on the cogwheels become greater, which may have a detrimental effect on the lifespan of the bearings.
Even with an optimum design of the meshing cogwheels, further influences are additionally involved in hydraulic devices through the operating medium, which have a negative effect on the noise development and the efficiency. The typical pressure pulsations in hydraulic cogwheel pumps, which are principally dependent on the number of teeth, the pressure difference between the inlet side and outlet side, and dynamic local pressure differences, may lead to a rebounding or vibrating of the teeth and therefore both to an undesired noise development and also to an unnecessary fluid reflux from the outlet side to the inlet side of the pump.
A hydraulic device including two meshing cogwheels, each cogwheel having external oblique teeth and being arranged between an inlet side and an outlet side is known from EP 0 769 104 B1. Excess pressure cut-outs (control grooves) and fluid supply cut-outs are provided on both end sides of the cogwheels, these cut-outs being respectively offset with respect to each other according to the oblique teeth gap. The excess pressure cut-outs are permanently connected with intermediate spaces between the teeth of the two cogwheels. Through this, fluid shall be able to escape to the outlet side from the intermediate spaces which become smaller during the rotation of the cogwheels, in order to avoid a fluid reflux to the inlet side.